Three configuration methods for FPGA

There are three primary methods for configuring an FPGA (Field-Programmable Gate Array):

1. Parallel Configuration (JTAG or SPI)
Method Overview: Parallel configuration is one of the most common methods to load the bitstream (the configuration file) into the FPGA. It involves using a dedicated configuration port on the FPGA, such as JTAG or SPI (Serial Peripheral Interface), to load the configuration data from an external device (such as a PC, microcontroller, or a dedicated configuration controller) into the FPGA.

JTAG (Joint Test Action Group):

JTAG is a standard used for debugging and programming digital circuits, including FPGAs.

The JTAG interface is used to load the bitstream into the FPGA. It’s a serial interface, but it allows the data to be shifted in parallel into the FPGA.

JTAG is often used during development and debugging stages because it allows for easy access and control of the FPGA configuration and debugging of the internal logic.

Advantages:

• Easy to use for initial development and testing.
• Supports device programming and testing.

Disadvantages:

• Slower than other methods for final deployment.
• Requires external hardware for configuration.

SPI (Serial Peripheral Interface):

In SPI configuration, a serial flash memory stores the bitstream file, and the FPGA loads the bitstream over the SPI interface.

The FPGA is connected to a SPI flash memory that contains the configuration data, and the data is transmitted serially to the FPGA on startup.

Advantages:

• Simpler and lower pin-count configuration.
• Can be used for in-system programming (e.g., microcontrollers or systems on chip can program the FPGA).

Disadvantages:

Slower than parallel configuration methods for large bitstreams.

2. Serial Configuration (QSPI, SPI Flash, USB, Ethernet)
Method Overview: Serial configuration involves transmitting the bitstream to the FPGA in a serial fashion, either from a local memory or external source. This method is typically used in embedded or mass production scenarios. The configuration file is usually stored in serial flash memory or can be transmitted via interfaces like USB, Ethernet, or SDIO.

Common Protocols:

• QSPI (Quad SPI): A faster form of SPI that uses multiple data lines (usually four) to increase the data transfer rate. QSPI is typically used in modern FPGAs for serial configuration because of its high data throughput.
• SPI Flash Memory: Stores the FPGA configuration bitstream and transmits the data to the FPGA during the boot process.
• USB / Ethernet: Can also be used for serial configuration, especially for remote configuration or when connecting to a development PC.

Advantages:

• Lower Pin Count: Fewer pins are required for configuration, which is useful for small or embedded systems.
• Faster than traditional JTAG: Especially when using QSPI or USB for higher speed transfers.
• Remote configuration: Allows configuration via communication ports such as USB or Ethernet, making it easier to configure FPGAs in remote locations.
• Simple System: Configuring from a single external memory module simplifies the design.

Disadvantages:

• Slower than parallel configuration methods for large bitstreams (though faster than traditional SPI).
• Might require an external bootloader or memory to store bitstreams.

3. Flash Memory Configuration (Onboard Flash, EEPROM, External Flash)

Method Overview: This method involves storing the FPGA’s configuration bitstream in non-volatile flash memory (e.g., EEPROM or flash chips) on the FPGA board. Upon powering up, the FPGA can automatically load the configuration data from the onboard flash memory, without requiring an external host computer or additional programming hardware.

How It Works:

• The FPGA has a dedicated configuration port (e.g., SPI, QSPI) connected to external flash memory (typically an SPI Flash or Parallel Flash).
• Upon power-up, the FPGA reads the bitstream from the external flash memory and configures itself. This process is often automatic and does not require manual intervention.
• Many embedded systems use this method, especially in cases where a stand-alone, autonomous system is required (e.g., IoT devices or edge devices).

Advantages:

• Autonomous operation: The FPGA configures itself at power-up without needing external intervention.
• Low-cost and compact: Using external flash memory to store the bitstream makes the design compact and cost-effective.
• No need for external configuration hardware: Once the system is deployed, there’s no need for additional programming tools.

Disadvantages:

• Limited flexibility: If the bitstream needs to be updated, you may need to access the flash memory and reprogram it, which can be more difficult compared to serial or parallel methods.
• Longer configuration time: Serial loading from flash can take time, especially for large designs.

Summary of Configuration Methods:

Conclusion:

• Parallel configuration is best suited for development and high-speed applications, but it's often not used in final production due to pin count and complexity.
• Serial configuration methods such as QSPI are widely used in embedded systems for their balance of speed and simplicity.
• Flash memory configuration is the most common method for standalone systems, as it allows the FPGA to be configured autonomously from non-volatile storage at startup.